1. Field of the Invention
This invention pertains to communications receiving systems designed to receive multiple burst transmissions, occurring on the same frequency and overlapping in time, using direct-sequence spread-spectrum (DSSS), code-division multiple-access (CDMA), phase-shift-keyed modulation. This type of transmission can be employed in a star-type communications network from the remote nodes to the central node, or in a mesh-type communications network between the nodes, and may or may not include relay stations such as those that would be used in satellite communications networks.
2. Description of the Related Art
Several transmission schemes have been presented for CDMA communications systems. The Telecommunications Industry Association IS-95 forward link (base-to-subscriber-terminal) scheme is an orthogonal synchronous CDMA system in which the spreading sequences used to communicate with different users are Walsh-Hademard functions. These highly orthogonal codes are used to minimize mutual interference between signals addressed to different users. However, there is a need for very precise synchronization to maintain orthogonality between signals transmitted to and by the various users. On the reverse link (subscriber-unit-to-base), each subscriber unit is assigned a unique spreading sequence. These spreading sequences are chosen to have good cross-correlation properties.
The C200 system, developed by Equatorial communications Company, uses an asynchronous scheme that provides one code per very small aperture terminal (VSAT). The spreading sequences used in this system are modified Gold codes. These codes have good asynchronous cross-correlation properties. However, since each VSAT has a unique spreading sequence and any VSAT may transmit at any time, each VSAT requires a dedicated receiver at the hub.
In a paper entitled "Fundamentals of Packet Multiple Access for Satellite Networks," by Norman Abramson (IEEE Journal on Selected Areas in Communications, Vol. 10, No. 2, February 1992), the author suggests a transmission scheme for VSAT terminals that he refers to as "spread ALOHA." In this scheme, all users would employ the same spreading sequence. This spreading sequence would be chosen to have a good autocorrelation peak and low autocorrelation sidelobes. The hub earth station would transmit a pilot signal to facilitate the synchronization of the transmissions from the various VSATs in the network. However, Abramson does not describe how to implement a hub earth station receiving system for multiple simultaneous inbound VSAT transmissions. Moreover, the capacity of such a system in terms of the number of simultaneous users depends heavily on the length of the spreading sequence employed (and hence the satellite transponder bandwidth required for a given inbound data rate, coding, and modulation technique) and how well the VSAT system can be synchronized.
To overcome the technical difficulties of the synchronization of various transmitters and the economic problems of requiring a dedicated receiver for each possible transmitter, a new CDMA transmission and reception scheme is needed. This scheme should operate like a conventional TDMA or FDMA scheme in that a set of receiving system resources can be shared between multiple users whose transmissions do not collide with one another. It should also not impose burdensome synchronization requirements for successful operation. It is therefore desirable to provide a system for properly routing incoming signals to a plurality of demodulators in a manner that will avoid the problems set forth above.